1. Field of the Invention
The present invention relates to a planar antenna of a relatively small size and a low input impedance, which is preferable for use as an antenna in a GPS (global positioning system), for example.
2. Description of the Prior Art
Generally, planar antennas that are of a simple and rugged structure, small in size, and of a low profile are widely known as antennas suitable for use in satellite communications and mobile communications.
Mostly, circularly polarized electromagnetic waves are used in satellite communications and mobile communications.
FIG. 1 of the accompanying drawings shows a conventional planar antenna according to the invention entitled "small-size microstrip antenna" that is disclosed in Japanese laid-open patent publication No. 2-48803. The disclosed planar antenna comprises a circular radiating conductor 1 having four deep notches 3.about.6 defined therein at 90.degree.-spaced angular positions and extending radially inwardly from the outer circular edge of the circular radiating conductor 1 toward the center thereof, the notches 3.about.6 being symmetrically arranged with respect to a feeding point 2. It is also described that the planar antenna of this structure has a low resonant frequency and is small in size, and that the radiating conductor 1 may be of a square shape rather than the circular shape.
It is desirable that planar antennas allow easy impedance matching with associated feeding systems and have a wide frequency band in certain fields of communications.
With the conventional planar antenna shown in FIG. 1, however, as the length of an offset of the feeding point 2 from the center of the circular radiating conductor 1 varies, the impedance varies to a relatively large extent, making it difficult to achieve impedance matching with a feeding system having an impedance of 50 ohms.